Humidity and Dewpoint - questions?

Question

My assumption with dewpoint is that the air holds a certain amount of water. If the relative humidity = 80%, that means that the air can only hold 20% more humidity before - well that's my first question. If the air is completely saturated with humidity, but it is a hot day and water vapour continue to evaporate, would that create fog? So essentially, would the Dewpoint be the same as + 100% humidity?

Answer 1

Dewpoint is a temperature at which the water in the air condensates.

A relative humidity (RH) of 100% means the temperature is equal to the dewpoint. So if dewpoint = 14C and temperature is 14C, the RH is 100%. But if dewpoint = 14C and temperature is 18C, the RH is about 80%.

About your questions. First, if the relative humidity is 80%, it means that the air is 80% 'full' with water (can hold 25% more than the present water mass).

Second, if the air is completely saturated with humidity, it simply can't hold more water. In this case, water can evaporate only if the same amount of water (from the air) condensates.

Fog is another thing - it is a cloud in contact with earth (tiny droplets of condensated water in the air).